In general, among the flow rate measuring devices, in a straight pipe type flow rate measuring device which directly measures a large flow rate, it is required to increase a diameter of piping because a flow rate is reduced to a measurable region of a flow rate detecting element. Thus, there was a limit in size reduction of the device. In particular, if the flow rate is increased, output linearity of the flow rate detecting element is lowered, and it becomes difficult to perform highly accurate measurement. Therefore, in order to improve the measurement accuracy as well as promote the size reduction, a small sized flow rate measuring device is proposed in which a primary flow channel is provided with a secondary flow channel, a flow rate of a fluid flowing through the secondary flow channel is measured, and based on the measured flow rate, a flow rate of the fluid flowing through the primary flow channel is converted.
Conventionally, as a flow rate measuring device provided with a secondary flow channel, for example, a flow rate module in which a primary flow channel is provided with a flow limiter is known (see Patent Document 1). Further, a combustion gas flow rate measuring device constructed of a first main pipeline block and a second main pipeline block is known (Patent Document 2).    Patent Document 1: Domestic re-publication of PCT international application No. 2003-523506    Patent Document 2: JP Patent No. 03340655